The invention relates to a computed tomography apparatus which includes a scanning unit with a radiation source and a detector unit which is connected thereto in order to detect a conical radiation beam, emitted by the radiation source, after its passage through the examination zone of a patient who is situated between the radiation source and the detector unit,
a drive unit for producing a relative motion, formed as a helix around an axis of rotation, between the scanning unit and the patient, and
a reconstruction unit for reconstructing a 3D image data set of the examination zone from the measuring data acquired by the detector unit within a detector window defined by the helix, the connecting lines from the radiation source to the two edges of the detector window, being offset relative to one another in the direction of the axis of rotation, intersecting two segments of the helix which are offset by the distance (2n+1)p in the direction of the axis of rotation, where n is a small integer number larger than or equal to 1 and p corresponds to the axial offset between two neighboring turns of the helix, so that the Radon domain is regionally filled with a different degree of redundancy by the measuring data.
A computed tomography apparatus of this kind (also referred to hereinafter as CT apparatus) is known from EP 0 981 995 A2. The scanning trajectory in this computed tomography apparatus is shaped as a helix and a conical radiation beam traverses the examination zone of an object to be examined, for example, a patient. In the cited publication it is also proposed to choose the dimensions of the detector window (or the part thereof which is used for the reconstruction) so as to be a factor of 3, 5, 7 . . . larger than the distance between neighboring turns of the helix. When such a geometry is chosen, each voxel in the examination zone is irradiated exactly from an angular range of 3π, 5π, 7π, . . . when it passes through the radiation cone. Consequently, the Radon domain is filled at least regionally with a multiple redundancy of, for example, 3, 5, 7 times. Such a data acquisition ultimately enables an enhanced image quality to be achieved.
The use of computed tomography for imaging in the cardiac region often gives rise to images containing artifacts which are due to the cardiac motion during the data acquisition. In order to reduce such artifacts, use is often made of reconstruction methods in which a cardiac motion signal, for example, an ECG signal, which has been additionally acquired during the acquisition of the measuring data is evaluated in order to base the reconstruction exclusively on the measuring data which has been acquired during cardiac phases with little motion. However, it must be ensured that an adequate number of data from such cardiac motion phases with little motion of the heart is indeed available for the reconstruction as otherwise reconstruction of a 3D image data set cannot be performed at all.
A further problem is encountered in that the examination zone of interest of a patient is larger than the volume which can be scanned by way of a circular trajectory. Therefore, helical trajectories are often used for the acquisition of the measuring data; in that case it is necessary to acquire measuring data with a degree of redundancy so as to enable the described reconstruction with evaluation of the cardiac motion signal (so-called gated reconstruction) to be carried out.